Process and device for the continuous treatment of silicon

ABSTRACT

The invention relates to a process for the continuous treatment of silicon in which a slag in a pivotable low-shaft furnace (1) with a discharge pipe (4) reaching the bottom of the furnace tank is taken to a temperature of 1450° to 1800° C. and this slag is used to melt solid silicon and/or liquid silicon is continuously refined and the liquid refined silicon is then sprayed with compressed air or nitrogen (7) and continuously conveyed into a transport crucible (11) by being poured into a stream of water (9) in the channel (8) via a dewatering filter (10) and thus obtained in granular form. The invention also relates to devices for implementing the process.

FIELD OF THE INVENTION

The present invention relates to a process and an apparatus forcontinuous treatment of silicon.

BACKGROUND OF THE INVENTION

Patent print DE-A1 3331046 describes a process for treating silicon andferrosilicon with slag. In this process a silicate smelt with thecomposition

    ______________________________________                                        K.sub.2 O            2 to 13% by weight                                       Na.sub.2 O           0 to 2% by weight                                        Σ K.sub.2 O + Na.sub.2 O                                                                     2 to 13% by weight                                       SiO.sub.2            45 to 72% by weight                                      Al.sub.2 O.sub.3     0 to 30% by weight                                       Σ SiO.sub.2 + Al.sub.2 O.sub.3                                                               60 to 78% by weight                                      CaO                  0 to 30% by weight                                       MgO                  0 to 30% by weight                                       Σ CaO + MgO    15 to 30% by weight                                      CaF.sub.2            0 to 10% by weight                                       MgF.sub.2            0 to 10% by weight                                       Σ CaF.sub.2 + MgF.sub.2                                                                      0 to 10% by weight                                       Σ CaO + MgO + CaF.sub.2 + MgF.sub.2                                                          15 to 30% by weight                                      ______________________________________                                    

and impurities due to the raw materials, is smelted and overheated in aresistance furnace. Solid, preferably low-grade, dusty silicon orsilicon waste is fed into this silicate smelt. At the end of themelt-down or refining process the silicon is tapped. The silicon thusobtained can be processed like lumpy silicon produced by smeltingmetallurgy.

This process works by batch processing. This has the disadvantage thatthe furnace charge comprising slag and melted-down silicon must begreatly overheated before tapping so that a high yield of silicon can beobtained. Unless previously overheated the furnace cools down during thefollowing charging of silicon or silicon dust to such an extent thatsilicon dust slags and yield is lost or the melt-down slag becomesuseless prematurely.

A further disadvantage of this process is that from tap to tap thedischarge must be burned out. This detracts from smelting time. Pour-outtime is also lost for the melt-down process. A further time loss iscaused by the charging of the silicon or silicon dust. Cooling of theslag by the cold silicon reduces the furnace efficiency by up to onehalf for a period of about 1/4 hour to 1/2 hour.

When liquid silicon is not smelted from silicon dust or waste siliconbut produced by the customary carbothermic reduction process it must besubjected after tapping to an elaborate refining process in a ladle toobtain the necessary purity of so-called chemical qualities. In therefining processes currently employed here, oxygen or oxygen/nitrogenmixtures are top-blown onto the silicon in the tapping receiver or blownthrough the silicon smelt to oxidize aluminum and calcium. The resultinglosses due to combustion, slagging and splashing are up to 15% of theweight of the tapped silicon. After being refined the silicon is pouredinto tubs like the silicon melted-down from dust or waste.

For carrying off the resulting large silicon plates or the tundishfilled with silicon to the crushing plant one requires heavy loadingmachines or high-lift trucks. For coarse-crushing the silicon plates orblocks, which are up to 500 mm thick, one requires several largecrushers arranged in tandem. The processing of the large-sized siliconis thus very costly and labor-intensive.

Laid-open print DE 36 10 257 describes a process for granulating slagsand molten baths such as silicon to obtain foamed slag or granularmaterial as large-sized as possible. This process is problematic forproducing granular silicon since hydrogen explosions can occur as soonas the ratio of the amount of poured-in silicon to the amount of waterused is not right or the temperature of the poured-in silicon is toohigh. Large amounts of hydrogen then develop immediately, involving thedanger of an oxyhydrogen explosion.

The invention is based on the problem of providing a continuous processfor smelting and/or refining and for continuously pouring andcoarse-crushing silicon which avoids the disadvantages of the prior art,and an apparatus for carrying out this process.

SUMMARY OF THE INVENTION

The other part of the problem is solved by a tilting electrically heatedlow-shaft furnace having a discharge pipe, and an apparatus forcontinuously pouring or granulating silicon comprising a launder forcollecting the silicon emerging from the furnace and retaining anyentrained slag, optionally nozzles for spraying the liquid silicon, andtroughs for carrying off or cooling the liquid or solidified silicon.

The inventive process uses a slag like the known process of DE 333 10 46A1. The slag suitable for the inventive process has the followingcomposition:

    ______________________________________                                        K.sub.2 O            0 to 15% by weight                                       Na.sub.2 O           0 to 10% by weight                                       Σ K.sub.2 O + Na.sub.2 O                                                                     2 to 15% by weight                                       SiO.sub.2            55 to 72% by weight                                      Al.sub.2 O.sub.3     0 to 15% by weight                                       Σ SiO.sub.2 + Al.sub.2 O.sub.3                                                               60 to 78% by weight                                      CaO                  0 to 35% by weight                                       MgO                  0 to 35% by weight                                       Σ CaO + MgO    15 to 35% by weight                                      CaF.sub.2            0 to 8% by weight                                        MgF.sub.2            0 to 8% by weight                                        Σ CaF.sub.2 + MgF.sub.2                                                                      0 to 8% by weight                                        Σ CaO + MgO + CaF.sub.2 + MgF.sub.2                                                          15 to 30% by weight                                      ______________________________________                                    

and impurities due to the raw materials.

The totals stated above are conditions which restrict the free selectionof the single components. The proportion of impurities due to the rawmaterials is generally in the range of 0.1 to 1.5% by weight. Allpercentages by weight add up to 100% by weight in each special slagcomposition.

The slag is located in a tilting furnace electrically heated by graphiteor coal electrodes and having a discharge pipe extending down to thebottom of the furnace. The temperature of the slag is 1450° to 1800° C.Silicon is fed into the molten slag in a solid or liquid form. The ratioof amount of slag to amount of silicon used is preferably 1.6 to 3.2:1parts by weight. After being fed into the slag the lumpy or small-sizedsilicon is converted to the molten state, thereby forming a liquidtwo-phase system with a bottom layer of molten silicon and molten slagfloating on top.

After the silicon is liquefied the discharge is burned out and thefurnace tilted about 10-17 angular degrees out of the horizontalposition so that the liquid silicon under the slag layer can be removedvia the discharge pipe. To the same extent as liquid silicon is removed,solid or liquid silicon is recharged to the tilted furnace. The liquidsilicon removed via the discharge pipe hits a launder which collects itand retains any entrained slag, and further troughs for carrying off orcooling the liquid silicon. This liquid silicon is preferably sprayed atleast partly and carried off by a sharp water jet or granulated andsolidified with the aid thereof, or the silicon is poured into apreferably cooled oscillating conveyor trough lined with suitablematerials.

It has surprisingly turned out that the liquid silicon can be sprayed onair with compressed air or preferably with an inert gas without largeamounts of silicon burning, which would immediately lead to an intensivedevelopment of SiO₂ smoke. The liquid silicon can surprisingly besprayed on air so easily that far greater smoke development can bedetected when liquid silicon is tapped into tubs or ladles in the normalway.

It has proved to be very advantageous to spray silicon by means ofcompressed air and/or nitrogen to a drop size of about 1-10 mm over atrough with water flowing through it, the fusion heat of the siliconbeing quickly removed in the fast flowing cold water. This mode ofoperation avoids the danger of a hydrogen-oxygen reaction.

The obtained granular silicon is dried and then ground. It already has agrain size of 90% smaller than 10 mm, is irregular and very brittle sothat this granular silicon can either be ground directly to the desiredfinal fineness of e.g. smaller than 0.3 mm or smaller than 0.5 mm, orneed only be sent through a fine crusher before grinding.

Despite its extremely quick cooling by being poured into water, thecoarse-crushed or granulated silicon metal produced by the inventiveprocess surprisingly shows no increased reactivity during silanesynthesis compared to silicon slowly solidified in customary tubs, asdescribed for atomized material in patent print DE 3 823 308. Unlike thesilicon produced by the inventive process, the so-called atomizedsilicon of patent print DE 38 23 308 is directly atomized from the smeltto finenesses smaller than 500 microns with a preferred particle-sizedistribution of 30 to 300 microns. According to the inventive processthe silicon is not to be atomized to grain sizes under 500 microns. Thesilicon produced according to patent print DE 38 23 308 is thus of atype different to the inventive material.

The inventive process saves the cost for crushing silicon plates. Thecost of inventively granulating or coarse-crushing the silicon is a merefraction of the cost of crushing silicon plates or blocks. Also, thegranular silicon or small silicon plates or heap of solidified silicondrops obtained by the inventive process are virtually dust- and thuswaste-free, while crushing silicon plates to a corresponding grainfineness gives rise to about 1 to 3% fine dust smaller than 70 micronsas waste.

For continuously melting down silicon waste and/or silicon dust andcontinuously purifying the resulting liquid silicon on the basis of theinventive process (the melt-down furnace variant) the following slagcomposition has proved particularly advantageous:

    ______________________________________                                        K.sub.2 O          2 to 10% by weight                                         Na.sub.2 O         0 to 2% by weight                                          Σ K.sub.2 O + Na.sub.2 O                                                                   2.5 to 10% by weight                                       SiO.sub.2          62 to 72% by weight                                        Al.sub.2 O.sub.3   0 to 10% by weight                                         Σ SiO.sub.2 + Al.sub.2 O.sub.3                                                             64 to 75% by weight                                        CaO                2 to 32% by weight                                         MgO                2 to 32% by weight                                         Σ CaO + MgO  16 to 32% by weight                                        Σ CaF.sub.2 + MgF.sub.2                                                                    max. 3% by weight                                          Σ CaO + MgO + CaF.sub.2 + MgF.sub.2                                                        16 to 32% by weight                                        ______________________________________                                    

The preferred temperature in this process variant is between 1600° and1700° C.

The described process can thus be used for example to recover superfinesilicon dust arising as a waste product from the grinding of lumpysilicon to a low-dust grain, as a coarse-crushed starting product forsilicon grains for fluid-bed furnaces for example.

When the process is employed to melt down silicon dust a 30 to 70%increase in melting efficiency is reached over the conventional mode ofoperation. At the same time the specific power consumption drops byabout the same measure. The cost for melting down low-grade silicon dustlikewise drops by this measure.

It is also within the scope of the invention to use the inventiveprocess for continuously purifying molten silicon as arises for exampleas the tapping from a reduction furnace. To carry out this processvariant (the refining furnace variant) one taps molten silicon forexample from a reduction furnace into an inventive furnace. Theinventive furnace contains a slag which is smelted and held by means ofelectric resistance heating. The temperature of the slag is preferably1470° to 1570° C. The following slag composition has proven particularlyadvantageous for continuously refining liquid silicon produced forexample in a reduction furnace on the basis of the inventive process:

    ______________________________________                                        K.sub.2 O          1 to 10% by weight                                         Na.sub.2 O         0 to 5% by weight                                          Σ K.sub.2 O + Na.sub.2 O                                                                   2.5 to 10% by weight                                       SiO.sub.2          62 to 72% by weight                                        Al.sub.2 O.sub.3   0 to 10% by weight                                         Σ SiO.sub.2 + Al.sub.2 O.sub.3                                                             64 to 75% by weight                                        CaO                2 to 32% by weight                                         MgO                2 to 32% by weight                                         Σ CaO + MgO  16 to 32% by weight                                        Σ CaF.sub.2 + MgF.sub.2                                                                    max. 3% by weight                                          Σ CaO + MgO + CaF.sub.2 + MgF.sub.2                                                        16 to 32% by weight                                        ______________________________________                                    

After being poured into the inventive furnace the liquid siliconcollects below the slag layer and is freed from the impurities, aluminumand calcium, by reacting with the slag. By tilting the inventive furnaceone preferably removes the refined silicon in the same amount as it isfed from the reduction furnace. The purifying operation takes place bythe inventive process so fast that one pass of the silicon emergingcontinuously from the reduction furnace through the inventive refiningfurnace filled with slag suffices to reduce the silicon impurities,calcium and aluminum, to at least the same values as in the refiningprocess with an oxygen/nitrogen mixture.

Should the aluminum content of the silicon smelt drop below the setvalue in the inventive melt-down or refining process it can be raised bycontinuous realloying. This can be done for example by coiling aluminumwire continuously into the discharge pipe of the inventive furnace. Thealuminum wire melts in this discharge pipe in the swelling-up siliconand thereby dissolves. If necessary a stirring spool can be used tohomogenize the aluminum in the liquid silicon.

The electric power fed to the refining furnace can be used to regulatethe temperature of the refining slag and thus the pouring temperature ofthe silicon, which is likewise important for the continuous pouringoperation to avoid overheating of the liquid silicon.

Overheated, liquid silicon attacks the cast iron lining of theoscillating conveyor troughs. It also increases the risk of anoxyhydrogen explosion when the liquid silicon is cooled in flowingwater.

Further, up to about 10% silicon dust or silicon waste, based on theamount of liquid silicon produced in the reduction furnace, can becontinuously added to the refining furnace and melted down during therefining process in accordance with the electric power of the inventiverefining furnace.

In comparison to prior art refining processes in which combustion,slagging and splashing losses can be as high as 15% of the weight of thetapped silicon, the silicon losses in the inventive melting down andrefining of silicon dust (melt-down furnace variant) are less than 4% byweight. In the inventive refining process in a carbothermic reductionfurnace (refining furnace variant) they are in the range of only 0.5 to2.5% by weight.

One difference between the described process variants is as follows. Formelting down silicon or silicon dust one expediently uses furnaces withat least 2 MW of power in order to permit economical processing of thesilicon dust, while when the process is used for purifying liquidsilicon from a carbothermic silicon reduction furnace it suffices to usevery small low-shaft furnaces with a relatively low smelting power ofe.g. 0.3 to 0.6 MW to be able to process the amounts of liquid siliconproduced in a silicon reduction furnace with the currently customaryelectric connect loads of 10 to 25 MW.

When the process is only used for purifying and continuously pouring theliquid silicon produced by a silicon reduction furnace one basicallyrequires only the electric power necessary for premelting the slag andholding it.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to FIGS. 1 to 4 various embodiments of the inventiveprocess will be explained in more detail by way of example.

FIGS. 1 and 2 show two embodiments of the melt-down furnace variant;

FIGS. 3 and 4 show two embodiments of the refining furnace variant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1: From melt-down furnace 1, with electrodes 3 immersed in slag 2,liquid silicon 5 flows continuously through discharge pipe 4 extendingdown to the bottom of the furnace onto launder 6 where it is collectedand freed from entrained slag. The discharge pipe 4 further has acoiling aluminum wire 4a. The liquid silicon is then atomized by meansof compressed nitrogen from nozzle 7 and conveyed in trough 8 with asharp water jet from nozzle 9 via drain screen 10 into transport vat 11.

FIG. 2: From melt-down furnace 1 with electrodes 3 immersed in slag 2liquid silicon 5 flows continuously through discharge pipe 4 extendingdown to the bottom of the furnace onto launder 6 where it is collectedand freed from entrained slag. The liquid silicon is then atomized bymeans of compressed air from nozzle 7 and conveyed in air-cooledoscillating conveyor trough 12 lined with cast iron plates intotransport vat 11.

FIG. 3: From the lip of silicon reduction furnace 13 the liquid siliconruns into refining furnace 14 and is then processed in the same way asthe silicon from the melt-down furnace in FIG. 1.

FIG. 4: From the lip of silicon reduction furnace 13 the liquid siliconruns into refining furnace 14 and is then processed in the same way asthe silicon from the melt-down furnace in FIG. 2.

The following examples will explain the invention further.

EMBODIMENT EXAMPLES Example 1

In a tilting one-phase low-shaft furnace lined with coal tampingmaterial, heated with two graphite electrodes with a diameter of 400 mmand having a graphite discharge pipe extending down to the bottom of anelliptic furnace tank, the furnace tank being 1.6 m wide, 2.4 m long and1.1 m deep, 5 t slag with the following chemical analysis was melteddown:

    ______________________________________                                        K.sub.2 O   2.98% by weight                                                   Na.sub.2 O  0.84% by weight                                                   SiO.sub.2   62.60% by weight                                                  Al.sub.2 O.sub.3                                                                          1.99% by weight                                                   CaO         18.98% by weight                                                  MgO         11.58% by weight                                                  ______________________________________                                    

and impurities due to the raw materials.

The furnace was operated by immersing the electrodes in the liquid slagwith an electric power of 2 MW. After the smelt was melted down andoverheated to a temperature of about 1650° C. the charging of thesilicon dust was begun. 2.5 t silicon dust was charged. After thisamount of silicon dust was melted down the smelt was taken to about1680° C. and the discharge then burned out by means of an electrode.

The furnace was now tilted about 13 angular degrees so that a partialamount of a few hundred kilograms of the liquid silicon located at thebottom of the furnace tank could rise up in the discharge pipe and flowout.

The furnace was left at this tilt and the charging of silicon dust begunagain.

The effluent silicon was poured onto a launder which retained some slagthat had risen up in the pipe during pouring. The silicon flowed downthe launder and was atomized at the end of the launder by means of anozzle out of which nitrogen flowed at a pressure of 5 bars. The glowingsilicon drops fell into a trough in which cold water was flowing. Theywere picked up by the water jet and transported in the trough via adrain screen into a transport vat. From there they were conveyed into adryer where they were freed from moisture. A granular silicon wasobtained which had a grain size smaller than 10 mm. The bulk of thesilicon grains had a diameter of 1 to 7 min. The granular material wasthen ground.

After 113 t silicon dust was melted down the furnace operation becameunsteady, indicating that the slag was exhausted. The charging ofsilicon dust was now ended, the silicon poured completely out of thefurnace and the entire slag then emptied into the slag tub.

The melt-down performance in the inventive process was 1.5 t silicondust per hour. Customary batch processing only obtains an averagemelt-down performance of 0.9 t silicon dust per hour. The increase inmelting performance in the inventive process over the prior art was thus67%.

The yield was 97%, compared to a yield of 93% in the melting down inbatches according to the prior art.

Example 2

In a tilting one-phase low-shaft furnace lined with coal tampingmaterial and having an elliptic tank, the dimensions being a length of100 cm, a width of 80 cm and a depth of 55 cm, 400 kg slag with thefollowing chemical analysis was smelted by means of two graphiteelectrodes:

    ______________________________________                                        K.sub.2 O   1.25% by weight                                                   Na.sub.2 O  4.66% by weight                                                   SiO.sub.2   64.00% by weight                                                  Al.sub.2 O.sub.3                                                                          1.22% by weight                                                   CaO         17.79% by weight                                                  MgO         10.06% by weight                                                  ______________________________________                                    

and impurities due to the raw materials.

This inventive refining furnace with an electric connect load of 0.5 MWwas located below the discharge of a carbothermic silicon reductionfurnace with an electric connect load of 15 MW which produced about 1 tsilicon metal per hour.

The refining furnace was followed by a 1 m long launder which led to anoscillating conveyor trough lined with air-cooled cast iron plates. Atthe end of the 8 m long oscillating conveyor trough there was atransport vat into which the glowing silicon metal could fall.

After the abovementioned slag was completely smelted and heated in therefining furnace to 1530° C. the latter was filled about 2/3 with slag.The discharge aperture of the reduction furnace was now burned out.

The liquid silicon ran into the refining furnace in a uniform jet fromthe lip of the reduction furnace. Just before the tank of the refiningfurnace was filled with slag and silicon its discharge pipe was burnedout by means of an electrode. The refining furnace was now tilted andsilicon flowed onto the launder and from there onto the oscillatingconveyor trough. Below the end of the launder there was a compressed-airnozzle which atomized the liquid silicon to a particle size of about1-10 mm before it hit the oscillating conveyor trough. On theoscillating conveyor trough the silicon drops were cooled to the pointthat they no longer stuck together, i.e. they were surrounded on theoutside with a layer of solidified silicon.

This silicon was caught in the transport vat at the end of theoscillating conveyor trough, where it could cool further. When a vat wasfull of silicon it was replaced by an empty vat and the silicon fed tothe mill after complete cooling.

Approximately every 20 minutes about 30-40 kg silicon dust and/orsilicon waste was added to the refining furnace. After 21 t liquidsilicon was refined and 2 t waste silicon melted down the furnaceoperation became unsteady. The discharge aperture of the reductionfurnace was now closed, the silicon and then the slag completely pouredout.

A new slag was then smelted and the refining process could be continued.

The yield of refined silicon was 98.8%, based on unrefined silicon fromthe reduction furnace.

We claim:
 1. A process for continuous treatment of silicon comprisingthe steps of:a) providing a tilting low-shaft furnace having a treatmenttank therein and a discharge pipe communicating with an interior of saidtreatment tank; b) placing one end of said discharge pipe adjacent abottom of said treatment tank such that said discharge pipe extends atan angle from the bottom of said treatment tank; c) inserting slag intosaid treatment tank; d) heating the slag in said treatment tank to atemperature of about 1450° C. to about 1800° C.; e) adding silicon insaid treatment tank for at least one of melting said silicon andrefining said silicon with said heated slag; and f) tilting said furnacesuch that said refined silicon flows through said discharge pipe and isdischarged from said furnace.
 2. A process of claim 1, wherein step e)comprises the further step of using solid silicon as said silicon addedto said treatment tank, subsequent to heating said slag, and meltingsaid silicon with said heated slag.
 3. A process of claim 1, whereinstep e) comprises the further step of using liquid silicon as saidsilicon added to said treatment tank subsequent to heating said slag. 4.A process of claim 1, further comprising the step of, followingdischarge of said silicon from said furnace, pouring said silicon into awater jet thereby solidifying said silicon into a coarse form.
 5. Aprocess according to claim 1, further comprising the step of, followingdischarge of said silicon from said furnace, spraying said silicon intoa water jet thereby solidifying said silicon into a coarse form.
 6. Aprocess of claim 5, wherein the spraying step further comprises usingone of compressed air and nitrogen to spray said silicon.
 7. A processof claim 1, further comprising the step of, following discharge of saidsilicon from said furnace, pouring said silicon onto an oscillatingconveyor thereby to solidify said silicon into a coarse form.
 8. Aprocess of claim 1, further comprising the step of, following dischargeof said silicon from said furnace, spraying said silicon onto anoscillating conveyor thereby to solidify said silicon into a coarseform.
 9. A process according to claim 8, further comprising the step of,following discharge of said silicon from said furnace, spraying saidsilicon into a water jet thereby solidifying said silicon into a coarseform.
 10. A process of claim 1, further comprising the step of using thefollowing composition:

    ______________________________________                                        K.sub.2 O            0 to 15% by weight                                       Na.sub.2 O           0 to 10% by weight                                       Σ K.sub.2 O + Na.sub.2 O                                                                     2 to 15% by weight                                       SiO.sub.2            55 to 72% by weight                                      Al.sub.2 O.sub.3     0 to 15% by weight                                       Σ SiO.sub.2 + Al.sub.2 O.sub.3                                                               60 to 78% by weight                                      CaO                  0 to 35% by weight                                       MgO                  0 to 35% by weight                                       Σ CaO + MgO    15 to 35% by weight                                      CaF.sub.2            0 to 8% by weight                                        MgF.sub.2            0 to 8% by weight                                        Σ CaF.sub.2 + MgF.sub.2                                                                      0 to 8% by weight                                        Σ CaO + MgO + CaF.sub.2 + MgF.sub.2                                                          15 to 32% by weight                                      ______________________________________                                    

as said slag.
 11. A process for continuous treatment of siliconcomprising the steps of:a) providing a tilting low-shaft furnacedefining a treatment chamber therein and having a discharge pipe locatedadjacent and extending at an angle up from a bottom of the treatmentchamber; b) placing a coiled aluminum wire inside the discharge pipe; c)inserting slag into said treatment chamber; d) heating the slag in saidchamber to a temperature of about 1450° C. to about 1800° C.; e) addingsilicon in said chamber for refining said silicon with said heated slag;and f) tilting said furnace such that said refined silicon flows throughthe discharge pipe and is discharged from said furnace.
 12. An apparatusfor continuous treatment of silicon comprising:a) a tilting low-shaftfurnace having a treatment tank therein and a discharge pipecommunicating with an interior of said treatment tank; b) one end ofsaid discharge pipe being located adjacent a bottom of said treatmenttank such that said discharge pipe extends at an angle from the bottomof said treatment tank; c) means for heating slag, added to saidtreatment tank, to a temperature of about 1450° C. to about 1800° C.; d)and means for tilting said furnace such that refined silicon flowsthrough said discharge pipe and is discharged from said furnace; wherebywhen slag is added to said treatment tank and heated in said treatmenttank to a temperature of about 1450° C. to about 1800° C. and whensilicon is added to said treatment tank, said silicon is at least one ofmelted and refined by said heated slag and said refined silicon flowsthrough said discharge pipe and is discharged from said furnace.